This invention relates to Phase Locked Loop (PLL) circuits. More specifically, it relates to a PLL circuit which incorporates a monolithic PLL.
PLL circuits typically consist of a phase detector, a filter, an amplifier and a Voltage-Controlled Oscillator (VCO) arranged in a loop circuit. The phase detector compares the phase of an input signal to the phase of the output of the VCO and causes the amplifier to output a signal which is a measure of the phase difference of the two signals. PLL circuits are typically used to provide a stable clock output. Other uses include tone decoding, demodulation, frequency multiplication, frequency synthesis and the regeneration of signals.
Early PLL circuits were implemented with discrete components and were complex. The complexity led to a reluctance to use PLL circuits, however, the development of monolithic PLL components, such as the Signetics 564, has alleviated the earlier reluctance. The monolithic components are relatively inexpensive and easy to use.
These monolithic PLL components, however, provide fixed circuitry which must be used, even if the circuitry is not particularly suited to the desired function. For example, the Signetics 564 provides a mixer stage which, if used, introduces modulations in the frequency. This results in frequency variations in the output of the PLL circuit. Another problem frequently encountered is the type of phase detector used. Most monolithic PLL components utilize a Type I phase detector wherein the phase of an analog signal is measured. In many cases, particularly at high frequencies, it is prefered to use a Type II phase detector which is driven by digital transitions.
For some of the more demanding applications, therefore, it is desirable to utilize a PLL having specially designed circuitry. In these cases, however, it has already been noted that the circuitry is complex. Furthermore, specialized components, such as a VCO, frequently are not suitable for these demanding applications. For example, off the shelf VCOs do not have the range of linearity required for high frequency applications.
A Phase Locked Loop having high frequency capabilities, a wide locking band and fast locking, therefore, is needed.